I'm pretty sure it hasn't been done, at least in the mainstream, but I can't see a good reason why it can't be done. Unless someone can tell me where the showstopper is, I'm tempted to have a go. I can see the limitations; we need to work during startup or shutdown, with our filesystems not mounted, and that limits what we can detect about the state of the phone, filesystems, backups, etc. Can anyone see a problem in the following:

1) Start with backupmenu and automate it, such that when booted with keyboard open, it asks no questions, takes a backup and reboots. This would initially cause a reboot-backup-reboot loop unless and until the keyboard is shut.

2) Add some kind of check into bootmenu.sh to break that loop, so that we only launch the automated backupmenu under a predetermined set of conditions. Since we can't mount up the filesystem and check whether we just took a backup, can we check if the time is between 0400 and 0402 in the morning? If it is, run our automated backupmenu, if it isn't then boot into maemo.

3) Automate a reboot at 0400 daily provided that we're on charge. Assuming I've left the keyboard open, /sbin/preinit launches bootmenu.sh, which launches the autobackupmenu based on the time of day. This takes a backup to the microSD card and then reboots, whereupon preinit or bootmenu.sh decides that it's gone 0402, so go ahead and reboot maemo.

4) Automate the business of cleaning up the old backups and rsync the lot over to a PC nightly.

Admittedly, it hasn't been done yet that I know of, which causes me to wonder if I'm missing something, but is there any reason that can't work?